1. Field of the Invention
The present invention relates to a spherical lens and an imaging device utilizing such spherical lens, and more particularly to a spherical lens of high performance of uneven medium, such as a spherical lens composed of concentric spheres of mutually different media, and an imaging device of a high resolving power and a wide imaging angle utilizing such spherical lens.
2. Related Background Art
A spherical lens of a single medium is already well known. In such spherical lens of a single medium, the refractive index is the only freedom for correcting the aberration for a given radius, and a medium of high refractive index is required for correcting the spherical aberration. However such spherical lens could not be used for the applications requiring a high resolving power, as the spherical aberration cannot be completely eliminated even with a medium of high refractive index. Also such lens could not be used for applications utilizing white light, since the medium of high refractive index generally has a high dispersive power, giving rise to a significant color aberration.
On the other hand, spherical lenses of unhomogeneous medium are disclosed for example in the U.S. Pat. Nos. 4,257,672, 4,327,963 and 4,557,566 and the U.S. patent application Ser. No. 027,737, now U.S. Pat. No. 4,848,882 etc., of the present Applicant.
However the spherical lenses disclosed in the above-mentioned U.S. patents do not have performance enough for the applications requiring a high resolving power.
Also the spherical lens disclosed in the above-mentioned U.S. patent application Ser. No. 027,737 now U.S. Pat. No. 4,848,882 shows a very high performance but requires a special pattern of distribution of refractive index inside the lens.
On the other hand, there have been designed various lens systems in order to expand the imaging angle of the image taking optical system. In such optical systems, an increase in the image size inevitably increases the lens diameter, thus giving rise to a bulky optical system. Also the conventional optical system, if designed for a wide imaging angle, will result in a significant decrease of the light intensity in the peripheral area, due to the well known cosine fourth power law.
The above-mentioned spherical lens has long been known as a simple optical system capable of achieving a wide imaging angle, but it has not been possible to obtain image information by focusing the image of an object for example on an image sensor such as a CCD through a shperical lens, since such spherical lens does not have a flat focal plane. For these reasons there has been desired an imaging device capable of achieving a high resolving power and a wide imaging angle.